Among conventional radar apparatuses, for example, JP 2002-40139A discloses a radar apparatus provided on a vehicle for radiating a transmission wave such as an optical wave or a millimeter wave in the forward direction and detecting a body located in front of the vehicle as a body reflecting the transmission wave based on the reflected wave. This radar apparatus is used for generating a warning when detecting a situation in which a distance relative to a preceding vehicle becomes short. In addition, this radar apparatus is also used for controlling the speed of the vehicle so as to maintain the inter-vehicle distance relative to a preceding vehicle.
In the vehicle radar apparatus, the radiation direction of a laser beam emitted by a laser diode is changed by using a polygon mirror, which is driven to rotate. The radar apparatus thus radiates a plurality of laser beams over predetermined angular ranges spread in the transversal direction and the vertical direction of the vehicle. When a beam-reflecting body reflects some of the laser beams, the reflected laser beams are received through a beam-receiving lens employed in the radar apparatus.
The reflected beams received through the beam-receiving lens are lead to a photo-sensitive device, which outputs a voltage signal representing the intensities of the received laser beams. Then, the distance between the radar apparatus and the beam-reflecting body is measured based on a interval of time lapsing since the radiation of the laser beams till the voltage signal reaches a reference voltage. In addition, the position of the beam-reflecting body in the transversal direction of the vehicle and the vertical direction is determined based on the radiation angle of the laser beam.
The beam-reflecting body, that is, an object to be detected by the radar apparatus of a vehicle, is another vehicle leading ahead of the vehicle. Usually, the preceding vehicle has a reflector for reflecting a transmission wave such as a laser beam on the rear face of the vehicle as a reflector with a high reflectance. In addition, the vehicle body of the preceding vehicle also has a relatively high reflectance even though the reflectance of the vehicle body is not as high as the reflectance of the rear face. Thus, the vehicle radar apparatus is capable of detecting a preceding vehicle at a distance in front of the vehicle by more than 100 meters.
However, the output intensity of a transmission wave such as a laser beam is limited by a variety of restrictive conditions. As a result, the output intensity determines the upper limit of the distance relative to a preceding vehicle to be detected.
When dirt or snow covers the rear face of a preceding vehicle, the intensity of a beam reflected by the rear face decreases. In this case, it is difficult to distinguish received signal components each having an intensity representing a beam reflected by the preceding vehicle from noise components attributed to a variety of causes. As a result, the distance which the conventional radar apparatus of a vehicle is capable of detecting a preceding vehicle is limited and not satisfactory.
USP 2004/0169840A1 proposes a vehicle radar apparatus to counter the above drawbacks. This proposed radar apparatus radiates a plurality of laser beams in a predetermined angular range spread in the transversal direction of the vehicle. The radiated laser beams are reflected by a beam-reflecting body such as a preceding vehicle as reflected laser beams represented by a plurality of received signals. The radar apparatus then finds an integration value of the received signals obtained in succession as a result of reflection of the successively radiated laser beams, which are adjacent to each other.
Thus, even when the intensity of reflection beams reflected by a beam-reflecting body decreases due to low reflective materials, the received-signal components representing the reflected beams are integrated as if they are amplified. As a result, even when the distance relative to a beam-reflecting body as a detection object such as a preceding vehicle increases or the intensity of reflection beams reflected by a beam-reflecting body decreases, desired detectable distance relative to a beam-reflecting body can be attained.
In a process carried out by the radar apparatus of a vehicle to find an integration value of a plurality of received signals, the radar apparatus shifts the group of received signals serving as an object of the integration over the range of radiation. Thus, it is possible to maintain the desired detectable angular resolution of the vehicle radar apparatus.
The integration-type radar apparatus for a vehicle is shown in FIG. 7. It is to be noted in FIG. 7 that 327 laser beams (B1 to B327) are radiated in a range spread in the transversal direction of the vehicle and the number of received signals to be integrated for a group of integration is four. The integration is carried out on received signals representing four successive laser beams adjacent to each other.
In addition, the group of received signals as an object of integration is shifted by an interval corresponding to one received signal at a time over the range of radiation so that all received signals are subjected to the integration process. That is, first, received signals representing laser beams with scan numbers (or beam numbers) of B1 to B4 are designated as received signals of a group of integration. Then, the group of integration is shifted over the radiation range to the right by an interval corresponding to one received signal so that the group includes received signals representing laser beams with beam numbers of B2 to B5. Thereafter, this process to shift the group of integration by an interval corresponding to one received signal at one time is carried out in the same way repeatedly till the group of integration covers received signals representing the four right-most mutually adjacent laser beams with beam numbers of B324 to B327.
In synchronization with the operation to sequentially designate received signals as signals included in the group of integration, a process to integrate the received signals included in the group of integration is carried out. The integration process is carried out as follows. As shown in FIG. 8, the four received signals i+1 to i+4 of the integration group are sampled with the same sampling timing by using an A/D converter and converted into digital values in an A/D conversion process. Then, an integration value of all the digital values for the sampling timing is found. This process to find an integration value of all the digital values is carried out for every sampling timing.
The A/D conversion process needs to be carried out because the received signals are each an analog signal. Since the received signals each represent one of four laser beams, the process to find the integration value of the digital values obtained as a result of the A/D processing carried out with the same sampling timing produces a result representing the four laser beams. From the radar-apparatus point of view, the digital values obtained as a result of the A/D processing carried out with the same sampling timing represent received-signal components of reflection waves reflected by a beam-reflecting body at the same distance. Thus, by integrating these digital values obtained as a result of the A/D processing, the S/N ratio of the received signals representing reflection waves reflected by a beam-reflecting body can be improved.
In order to carry out the integration process of finding an integration value of digital values obtained as a result of the A/D processing for every sampling timing as shown in FIG. 8 for a sampling frequency of 50 MHz, for example, it is necessary to perform the integration process of finding an integration value of four digital values obtained as a result of the A/D processing at a frequency of 50 MHz or at intervals of 20 ns.
It is to be noted that, when a laser beam is used as the transmission wave, the distance from the radar apparatus to a beam-reflecting body is a half of a total distance, which is traveled by the laser beam since the time the beam is radiated till the time the beam reflected by the beam-reflecting body is received by the radar apparatus. Thus, in the case of an A/D conversion sampling frequency of 50 MHz or a sampling interval of 20 ns, the detected-distance resolution can be found as one half of the product of the velocity of the laser beam and the sampling interval. Since the velocity of the laser beam is 0.3 m/ns, the detected-distance resolution can be found as follows.0.3 m/ns×20 ns/2=3 m
In an operation to detect a beam-reflecting body such as a preceding vehicle running ahead of the vehicle having the radar apparatus, a distance resolution corresponding to an interval of 3 m is rather a coarse or rough distance interval. Thus, in order to improve the distance resolution, it is necessary to increase the sampling frequency to a value higher than 50 MHz.
The integration processing is carried out repeatedly as many times as the A/D-conversion and sampling processes of received signals in order to complete the integration processing for one group of integration. Since there are a large number of integration groups, the processing load of execution the integration processing is extremely heavy. In the example shown in FIG. 7, the number of integration groups is 324.
In order to counter this problem, in the integration-type radar apparatus, for a given sampling frequency, the detectable distance relative to a beam-reflecting body such as a preceding vehicle is basically determined by a sampling-point count of the A/D-conversion. The sampling-point count is defined as the number of sampling points in a sampling interval, which is the reciprocal of the sampling frequency. In the case of an A/D-conversion sampling-point count of N and a sampling process is started at the same time as the radiation of a laser beam, for example, for a sampling frequency of 50 MHz, a wave reflected by a beam-reflecting body at a distance of up to 3N (m) can be detected as a received signal. Thus, when it is desired to detect a preceding vehicle at a distance of up to 150 m, it is necessary to carry out a sampling process N (=50=150/3) times.
Since the sampling-point count of the A/D-conversion determines the number of integration processes for each group of integration, the sampling-point count has a great effect on the processing load. When the processing load is reduced by simply decreasing the sampling-point count, however, the detectable distance also becomes shorter due to the decreased sampling-point count. In turn, a short detectable distance leads to low detection performance of the radar apparatus. When the vehicle having the radar apparatus travels at a high speed, the vehicle needs to detect a preceding vehicle at a distance of at least 100 m in some cases. Thus, the detection performance of the radar apparatus must be improved.